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672. PARTI: A Program for the Computation of Thermodynamic
Properties and the Re-finement of Canonical Ensemble
Partition Functions in Flexible Molecular Sys-tems
by Alfonso Ni\244o and Camelia Mu\244oz-Caro, E. U.
Inform\240tica de Ciudad Real, Universidad de Castilla-La
Mancha, Ronda de Calatrava 5, 13071 Ciudad Real, Espa\244a
PARTI is a program that permits the calculation and
refinement of canonical partition functions using
different models for rotation and vibration. PARTI is
well-suited for the study of flexible, nonrigid
molecules. The program permits the computation of
thermodynamic properties.
The effect of overall rotation on the partition
function, Q, can be introduced using the usual semi-
classical expression or the direct summation of energy
levels. In this case, the program computes
variationally the rotational energy levels, which are
stored in a dynamic list. PARTI also permits one to
use either a rigid or a semirigid model to describe
rotation. In a similar way, the vibrational
contribution to Q can be obtained using the closed
formula for a harmonic oscillator or using direct
summations of vibrational energy levels. In this last
case, the vibrational levels must be input as data.
This option is useful for introducing the effect of
large amplitude vibrations. In the present im-
plementation, the program can compute 1nQ, d(1nQ)dT
and d2(1nQ)/dT2 as a function of T, working at constant
pressure. The results are three polynomial fits as a
function of T in the range T=273.15-373.15 K.
However, the program is prepared for a later extension
to the case of variable pressure.
The program is able to refine the initial fits of 1nQ,
d(1nQ)/dT and d2(1nQ)/dT2 using experimental in-
formation. The experimental data are used to build an
error function that is minimized using a quasi-Newton
algorithm. The results are refined expressions for 1nQ
and its derivatives that can be used to compute
thermodynamic properties not easily amenable to
experiment.
The different closed formulas (in the rigid rotor-
harmonic oscillator model) for translational, rotational
and vibrational contributions to the partition function can
be found in: (a) K. Lucas, Applied Ther- modynamics,
Springer Verlag (l991) and (b) D. A. McQuarrie,
Statistical Mechanics, Har- per & Row (1973). The
methodology used in the program can be found in: A. Ni\244o
and C.Mu\244oz- Caro, Computers Chem. (submitted for
publication).
Lines of Code: 6634
FORTRAN 90 NOTE: This program will not compile
under FORTRAN 77.
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